Small craft harbors with either fixed or floating docks require a relatively calm water environment, lest the moored vessels become damaged by wave-induced impacts with each other and/or the mooring facilities. A ‘relatively calm’ wave environment is understood to be one permitting a maximum of one and one-half feet in wave height in a 100-year storm event. To achieve this state of protection, breakwaters have been used to prevent entry of wave energy into boat harbors. Typically, unless they are of the rock and earth fill variety, called “rubble-mound”, these structures fall into two categories: floating and fixed.
Floating breakwaters (which are more properly referred to as “attenuators”) have always had serious limitations. Their effectiveness has been a function of their sheer size (width and depth) as it relates to the wave they are meant to block, moderate or attenuate. Unless they are of deep draft (on the order of one-half of the water depth), or extremely wide in a wave direction of travel (at least one-half of the wave length), floating breakwaters have been only marginally effective in reducing transmitted wave energy to acceptable levels.
A “good” floating attenuator is said to be one that transmits no more than 50 percent of the incoming wave energy. A wave attenuator's transmission coefficient (i.e., a ratio of a wave's energy after interacting with the wave attenuator to the wave's energy before interacting with the wave attenuator) provides one measure of the wave attenuator's effectiveness. A wave attenuator having a transmission coefficient less than about 0.50 (i.e. a wave's energy after interacting with a wave attenuator measures less than about 50% of the wave's energy before interacting with the attenuator) has historically been considered an effective wave attenuator.
In tidal conditions, the historically poor performance of floating wave attenuators has been exacerbated by the effect of increasing and decreasing water level on attenuation performance. For example, a float drawing six feet at a low-tide water depth of 12 feet (one-half of the water column) will be unable to provide the same degree of protection at a high-tide water depth of, say 25 feet (one-quarter of the water column). Accordingly, effectiveness of wave attenuation at high tide has been significantly diminished (e.g., compared to the effectiveness at low-tide).
In addition, wave period can vary with tide, even when the waves stem from a same-speed wind. Wave periods (and wavelengths) tend to be longer over deep water than shallow water. Thus, a floating wave attenuator having a width of about one-half of a wavelength at low tide will extend over the surface by a much lower percentage of the wavelength at high tide. Previous floating wave attenuators have thus had poor wave attenuation effectiveness under high-tide conditions.
In other words, previous floating breakwaters and wave attenuators might perform acceptably at low tide, marginally at middle tides, and imperceptibly at high tide.
Because such floating structures have typically needed to be of considerable draft and surface width (compared to mooring structures), floating wave attenuators have been retained using underwater anchoring, rather than vertical piles penetrating into the seabed (such as is common with mooring structures).
As used herein, “seabed” means an underwater, earthen surface underlying any type of body of water (e.g., a sea, a river, a lake, etc.).
Anchor systems typically permit the float a measure of lateral movement in response to winds and tidal currents, and thus have provided lower performance than pile anchored or fixed attenuators, regardless of tide. Known anchoring systems having taught anchoring lines at higher tides typically provide increased lateral motion at lower tides, due at least in part to slack anchoring lines at low tide. Such lateral motion can even generate waves within the harbor.
Thus, conventional engineering wisdom dictates that floating wave attenuators are of little practical use when wave heights exceed about three feet, and wave periods exceed about three seconds. Such conditions are typically generated around boat harbors where sustained wind speeds can reach (or even exceed) 30 miles per hour over a fetch of several miles.
Accordingly, engineers often turn to fixed panel breakwaters to provide a suitable level of protection for small boat harbors. Since a panel is by definition thin structure, panels typically have a large draft (e.g., extend downwardly to near the seabed). Some block as much as 80 percent (or more) of a vertical water column. Such fixed panels must extend upward, well beyond the highest tide level, to prevent overtopping by large waves at high tide. Consequently, it is not at all uncommon for such walls to tower 25 feet, 30 feet and even more than 40 feet above the lowest tide level.
Fixed panel breakwater designs usually involve treated timber, steel sheet or concrete slab structures fastened to piles driven into the seabed. At low tide, these wall panels obstruct vision, presenting a safety issue, as vessels entering the harbor have little time to react to other boat traffic. Further, such fixed panels create an unsightly, often smelly, assemblage of flotsam and sea-life, casting a sunlight-blocking shadow which hinders the growth of desirable sea grasses.
Over time, marine borers, decay, rusting fasteners, failed welds and weakened concrete with rusting reinforcement accelerate the diminution of the economic life of such breakwaters and wave attenuating structures (normally considered to be 15 to 20 years) without almost constant maintenance.